1. Field of the Invention
This invention relates generally to a developing device. More particularly, it is concerned with a developing device, in which a uniform thin layer of a single component developer is formed on a developer holding means, and then the thin layer of developer is disposed in confrontation to a latent image holding member to develop the latent image.
2. Description of Prior Arts
There have heretofore been known various types of developing devices, which utilize a single component developing agent.
Of these various types, a toner transfer development has been known to be a unique method. According to this method, a single component developer is applied onto a developer holding means in the form of a uniform thin layer, then this thin developer layer is disposed in confrontation to the surface of an electrostatic latent image with a small space gap therebetween, and the developer is caused to migrate from the developer holding means to the electrostatic latent image surface by the electrostatic force of attraction, thereby developing the latent image (vide: U.S. Pat. No. 3,232,190). According to this method, since the developer is not attracted to a non-image portion where no latent image potential exists and also the developer is not in contact with the non-image portion, a favorable development free from fogging can be effected. Further, since no carrier particles are used, there is no possibility of variations in the mixing ratio of the developer, hence no possibility of deterioration in the property of the carrier particles.
The assignee of the present invention has also filed U.S. Patent Application Ser. Nos. 938,101, 938,494, and 58,434, in which different types of the toner transfer development method are proposed.
According to the former two methods, a single component magnetic developer, a developer holding means made of a non-magnetic material, and a magnetic field generating means are arranged in the order as mentioned, and, after the developer is fed onto the developer holding means, a uniform thin layer of the developer is formed on the developer holding means by the magnetic force generated by the magnetic field generating means and a developer layer thickness regulating member. This thin layer of the developer is disposed in confrontation to the surface of an electrostatic latent image with a small space gap being provided therebetween so as to avoid contact at their surfaces. The development is effected by stretching out the developer at positions facing the image portions due to the electrostatic force of attraction. In this case, too, since the development is done in a state of the developer not being in contact with the non-image portion, a developed image perfectly free from the fogging can be obtained.
The latter development method is as follows. A single component magnetic developer, a developer holding means made of a non-magnetic material, and a magnetic field generating means are arranged in the order as mentioned. After the developer has been fed onto the developer holding means, a uniform thin layer of the developer is formed on the developer holding means by the magnetic force generated by the magnetic field generating means and a developer layer thickness regulating member. This thin layer of the developer is disposed in confrontation to the surface of an electrostatic latent image with a small space gap being provided therebetween so as to avoid contact at their surfaces. The development is effected by applying an alternating bias voltage as the developing bias voltage between the electrostatic latent image surface and the developer holding means, and by further changing the space gap with lapse of time. According to this development method, the developer is applied even to the non-image portion of the electrostatic latent image at the initial stage of the development, whereby a half tone portion of the latent image is developed, and, thereafter, the developer is applied only to the image portion with lapse of time. This latter method is effective in obtaining, in comparison with the former development method, a developed image having good reproducibility in the half tone portion and being perfectly free from the fogging.
Thus, according to the developing method wherein the thin layer of the single component developer (hereinafter called also "toner") is disposed in confrontation to a latent image, there can be attained excellent effects in its developing capability, image reproducibility, life of the developer, and so forth, in comparison with the conventional developing method.
However, even with these developing methods, there are various problems to be mentioned hereinbelow in putting them into practice.
(1) Foreign substances get in between the non-magnetic developer holding means and the developer layer thickness regulating member, and the portion where the foreign substances exist do not allow the toner to pass therethrough, whereby no toner can be fed to this portion and streaky irregularity occurs in the toner layer. Most of these foreign substances are paper dust and thread dust which have intruded from outside. In other cases, such foreign substance might be agglomerated mass of the toner particles. Further, depending upon the quality of the toner, it tends to be fused onto the surface of the magnetic blade as the developer layer thickness regulating member, and to grow bigger with lapse of time to cause an irregular surface on the edge part of the magnetic blade. Although the occurrence of this fusion of the toner is largely governed by the material quality of the toner, when the number of revolutions of the non-magnetic sleeve as the developer holding means increases, the force of the toner to collide against the magnetic blade also increases with the consequence that the toner tends to be readily fused to the blade surface. While intrusion of such foreign substances and occurrence of the agglomerated toner due to its fusion are not so high in their frequency of generation, such intrusion and agglomeration, however small in quantity they may occur at one time, accumulate in the developing device little by little during use of the device over a long period of time, and such accumulated effect appears on the developed image.
As measures against such unfavorable phenomena, there has heretofore been made a proposal, if any, to merely construct the developing device in a closed structure to prevent dust, etc. from entering into the device. In most cases, the toner within the developing device is totally taken outside the device, and the dust accumulated on the blade is removed every time the intrusion of the foreign substances and the fusion of the toner occur. Such method, however, not only takes time, but also causes considerable staining of the device, etc. due to scattering of the toner particles here and there, hence an improvement in this respect has been desired.
On the other hand, when the foreign substances accumulated on the edge of the magnetic blade are going to be removed by an external mechanical means without taking the developer out of the container vessel, there often takes place such a situation that the toner is often urged toward the surface of the developing sleeve which is the developer holding means with the consequence that the toner particles are agglomerated, or the toner mass is tightly adhered onto the sleeve surface to invite adverse effects.
(2) During a long period of developing operation, a thin layer of the developer is formed on the surface of the developer holding means to lower the developing capability.
At the time of the developing operation, the developer constantly repeats contact and separation to and from the surface of the developer holding means, on account of which the surface of the developer holding means tends to be stained with the developer, or a film layer of the developer tends to be formed. This thin layer of developer may be sometimes a resinous component of a low molecular weight contained in the developer, or in other cases, a controlling agent to impart the triboelectricity to the developer, or, in still other cases, very fine particles of the toner that have not contributed to the development. In any event, when these components cover the surface of the developer holding means, in particular, when the developing method which utilizes the charge of the developer, the charge quantity of the developer falls short to cause a lowering in the developing density, deterioration in the image reproducibility, and various other problems.
(3) During a long developing operation, the developer particles agglomerate to increase the adhesive force between the developer and the surface of the developer holding means, whereby formation of the developer layer in the uniform thickness becomes difficult, and irregularity in the image development takes place thereby.
No problem occurs so far as the developer coated on the developer holding means contributes to the development within a short period of time and then separates from the surface of the developer holding means. However, when the developer which has not contributed to the development due to low image density, and other reasons remains on the surface of the developer holding means for a long period of time, the developer particles increase their mutual agglomerating force to partially form a thicker coated layer on the surface, thereby causing irregularity in the developed image.